1. Field of the Invention
The present invention relates to a non-polarizing beam splitter. More specifically, the invention relates to the non-polarizing beam splitter having an Au layer to be used in an optical device such as an optical pickup.
2. Description of the Related Art
Non-polarizing beam splitters branch incident light into transmission light and reflection light by a predetermined ratio regardless of a polarized light state, in a certain incident angle and wavelength. Such non-polarizing beam splitters can be manufactured by forming dielectric layers in a laminating manner, but dozens of dielectric layers should be laminated. For this reason, productivity is not good and it is technologically difficult to form dozens of layers with high accuracy. Further, it is difficult that reflectance is made to be approximately 50% or more by the laminated structure of the dielectric layers in a state that reflectance and transmittance of P polarized light and S polarized light are not changed.
Japanese Patent Application Laid-Open No. 60-113203, therefore, proposes a non-polarizing beam splitter where a first dielectric layer, an Ag layer and a second dielectric layer are formed on a slant face of a right angle prism and layer structures of the first dielectric layer and the second dielectric layer are symmetrical with respect to the Ag layer. Ag, however, easily corrodes, and thus it changes in quality due to water or the like to be used in the step of manufacturing non-polarizing beam splitters. For this reason, non-polarizing beam splitters using the Ag layer have a problem of decay durability.
On the other hand, Japanese Patent Application Laid-Open No. 61-11701 proposes a non-polarizing beam splitter using an Au layer instead of an Ag layer. Since Au is certainly a stable material which does not corrode, the problem of the decay durability is resolved. Since Au, however, is soft, it is easily scratched at the time of handling and machining. Au absorbs light in a visible light range, and it is used limitedly to the light in infrared regions in the non-polarizing beam splitter of Japanese Patent Application Laid-Open No. 61-11701.
The present invention is devised in view of such conventional problems, and its object is to provide a non-polarizing beam splitter using Au which prevents the Au layer from being scratched at the time of handing and machining, prevents peeling of the Au layer from a prism, and can cope with light in a visible range of wavelength of 640 nm to 820 nm to be used in DVD (wavelength: in the vicinity of 650 nm, 640 nm to 680 nm) and CD (wavelength: in the vicinity of 780 nm, 760 nm to 810 nm).